Insulating glass units (IGU) generally have two panels of glass separated by a spacer and require bedding glaze, sealant or adhesive to bond the glazing to structural elements comprising the unit. The two panels of glass are placed parallel to each other and sealed at their periphery such that the space between the panels, or the inner space, is completely enclosed. The panels are secured to the window frame assembly of the IGU by an adhesive or sealant that forms a “bedding” between the panels of glass and frame of the unit.
Various compositions and methods of glazing are known, e.g., back-bedding sealing is a method where the sash frame of the window frame assembly is placed horizontally on a back-bedding machine which provides a continuous bead of fluid back bedding sealant along the glazing leg of the IGU. In this method, the back bedding material creates a seal between IGU and the sash frame. Although additional equipment is required, this process allows the use of a variety of materials, including silicone sealants and adhesives that have advantageous performance characteristics.
Various types of sealants are currently used in the manufacture of insulated glass units including both curing and non-curing systems. Liquid polysulphides, polyurethanes and silicones represent curing systems, which are commonly used, while polybutylene-polyisoprene copolymer rubber based hot melt sealants are commonly used non-curing systems.
Liquid polysulphides, polyurethanes and silicones may be one component as well as two component systems. The two component systems comprise a base and a curing agent that are mixed just prior to application to the glass. Two component systems require a set mix ratio, two-part mixing equipment and cure time before the insulating glass units can be moved onto the next manufacturing stage.
Current two-part RTV silicone sealant compositions which are typically opaque, while effective due to the speed of cure still present some problems related to productivity since squeeze out is not invisible and requires clean up. Typically, opaque two-part RTV silicone compositions have one component that contains silanol-terminated diorganopolysiloxane and calcium carbonate filler and another component containing an alkyl-terminated diorganopolysiloxane, catalyst, cross-linker and adhesion promoter. Fumed silicas are not typically used in the component that contains the silanol terminated diorganopolysiloxane due to the tendency of the free silanol (—SiOH) groups on the fumed silica to interact with the silanol terminated polymer thereby causing the component to increase viscosity (structuring) during storage. Moreover, this structuring phenomenon limits the utility of fumed silica fillers in two-part silanol terminated diorganopolysiloxane based sealants.
As a result of this limitation a need therefore exists for translucent two-part RTV silicone rubber-forming compositions with improved stability providing rapid primerless adhesion, to a wide variety of substrates, e.g., window assembly, along with excellent physical properties.